There have been five dramatic leaps in electronic technology during the past hundred years. The first four include the light bulb, electron tube, transistor, and integrated circuit. The most recent jump, and perhaps the most significant, has been the emergence of the microprocessor in the 1970's and 1980's. A microprocessor and associated memory and interfacing components form a microcomputer, a physically small digital machine as powerful as a room-sized computer of only two decades ago. These microcomputers have found almost limitless applications from electronic games, calculators, microwave ovens, and point of sale terminals, to stop-and-go lights, automobile ignition controls, copying machine controls, and deep space probes. The microcomputer has been a leading factor in the proliferation of automated processing controls including industrial robots. However, there have been few microprocessor applications to electric power transmission and distribution systems. In particular, there has been nothing comparable to "automatable" control of the protective and operating apparatus used on power systems. The emergence of the microcomputer provides the technical capability to produce just such controls.
A control based on microprocessors will perform the functions of those formerly produced in a more economical manner and allow the additions of functions which were economically or technically unfeasible.
Reclosers are inserted into power lines to protect a power distribution system. Most faults on power distribution lines are of the momentary nature and of sufficient magnitude to blow fuses if allowed to be conducted to them. When a fuse does blow in a power distribution system, it is necessary to send somebody to change it which is a somewhat expensive proposition. A primary function of a recloser is to save fuses. In general this is done by sensing the peak value of the current conducted and interrupting its flow by opening or tripping a recloser before the fuses can blow. After an interval the recloser closes restoring power to the system where it remains closed until the next fault is sensed. The rate at which a fuse will blow and interrupt current is a function of the thermal heating of the fusible element. The rate of thermal heating is proportional to the power generated by the fault and each fuse has a time current characteristic which describes the time interval required to interrupt the fault current. The time interval is generally approximately inversely proportional to the value of the root mean square of the fault current. It is desireable to coordinate the recloser with the fuses to be saved to insure that the recloser in fact interrupts temporary fault currents before the fuses to be protected are blown. This is generally done by approximating the root mean square value of the fault current by sensing its peak value. It must also be recognized that all faults which occur on a power distribution line are not temporary, such as those caused by a branch momentarily falling against the line. Some faults are of a more permanent nature such as those caused by a line falling to the ground. As a consequence, reclosers are built so that they will only trip a limited number of times within a short duration before locking open. Were this not done a recloser would cycle until failure and many of the fuses to be protected would blow anyway. At some magnitude of fault current it is desireable to have the recloser open immediately to protect the line rather than following a time current characteristic. At intermediate levels it may be desireable from the power distribution stand-point to allow the fault current to flow for a limited period to allow the fault to burn itself open or blow the fuse. Many reclosers have alternate time current characteristics which achieve this goal. Typically a recloser will allow two shots or trip operations to follow a fast time current characteristic and two additional shots along a somewhat slower time current characteristic before locking open or out.
The advantages to be gained in building a microprocessor based recloser control are many and several of them are well recognized. The principle reason it has not been previously done is that a microprocessor based microcomputer is a comparatively low power, low voltage, low current device which could not survive much less reliably operate in the hostile environment surrounding power distribution controls such as reclosers.